Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces of varying widths, the bars and spaces having differing light reflecting characteristics. Some of the more popular bar code symbologies include: Uniform Product Code (UPC), typically used in retail stores sales; Code 39, primarily used in inventory tracking; and Postnet, which is used for encoding zip codes for U.S. mail. Bar codes may be one dimensional (1D), i.e., a single row of graphical indicia such as a UPC bar code or two dimensional (2D), i.e., multiple rows of graphical indicia comprising a single bar code, such as a PDF417 and DataMatrix bar codes.
Systems that read bar codes (bar code readers) electro-optically transform the graphic indicia into electrical signals, which are decoded into alphanumerical characters that are intended to be descriptive of the article or some characteristic thereof. The characters are then typically represented in digital form and utilized as an input to a data processing system for various end-user applications such as point-of-sale processing, inventory control and the like.
Bar code readers that read and decode bar codes employing imaging systems are typically referred to as imaging-based bar code readers or bar code scanners. Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements (photosensors or pixels). An illumination apparatus or system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a system of one or more lens of the imaging system onto the pixel array. Thus, the target bar code within a field of view (FOV) of the imaging lens system is focused on the pixel array.
The pixel lines of the rolling shutter sensor are sequentially exposed and clocked out of the pixel array. The pixels of a global shutter sensor are simultaneously exposed and clocked out of the pixel array. In both cases the pixel values form a signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals representative of the captured image frame and attempts to decode the imaged bar code.
There are two requirements that increase the difficulty of successfully utilizing an imaging-based bar code reader. The requirement is being able to successfully image and decode increasingly higher density 2D bar codes. Increasingly higher density 2D bar codes are being utilized to provide an increasing quantity of encoded data in a given size bar code area or footprint. For many items, the available area where a bar code may be imprinted is limited. Thus, if more information is desired to be encoded in a bar code, bar code density must be increased, hence the trend from 1D to 2D bar codes. Increasing the density of a 2D bar code requires increasing the number of bar code features (bars or spaces) in a given area. However, deceasing feature size leads to problems in imaging resolution and decoding of high density 2D bar codes, such at PDF 417 and DataMatrix bar codes which utilize very small feature size. For example, the size of features of typical PDF 417 bar codes found on many state drivers licenses may be on the order of 4-5 mils, while the overall size of the bar code is relatively large, e.g., over one square inch. Obviously, the higher the density of a target bar code, the more difficult it is for the imaging system to generate an image with sufficient clarity and resolution to permit successful decoding of the target bar code image. If the bar code is moved closer to the reader imaging system, a size of the field of view FOV may become too small to image the entire bar code. On the other hand, if the bar code is move away from the reader imaging system, the resolution of the imaged bar code may be inadequate to allow for successful decoding of the imaged bar code.
The second requirement is the ability to image and decode a bar code being moved through the FOV of the imaging system. This is referred to as “swipe scanning”. Because of the movement of a target bar code within the FOV during imaging, the resulting image of the bar code may exhibit distortion that renders the decoding process more difficult or impossible. Generally, bar codes suitable for swipe scanning are lower resolution 1D bar codes which are easier to image and decode.
Characteristics of imaging systems that provide for the high resolution imaging necessary with high density 2D bar codes are different and often at odds with the characteristics of imaging systems that provide for successful imaging of moving, as opposed to stationary, bar codes. What is needed is an imaging-based bar code reader that includes an imaging system that provides for successful imaging of both high density bar codes and moving bar codes. What is also needed is an imaging-based bar code reader that provides such an imaging system in a housing that is capable of use in both a stationary mode or fixed position wherein a target bar code may be swiped or moved through the FOV of the imaging system and in a hand-held or portable mode wherein the reader may be picked up and moved about by a user to image and decode target bar codes on stationary objects, e.g., products on a shelf in a store or warehouse.